1. Field of the Invention
The present invention relates generally to heterojunction bipolar transistor structures, but more particularly it relates to heterojunction bipolar transistor structures having both cubically and hexagonally structured silicon carbide and/or SiCAlN electrically active regions.
2. Description of the Prior Art
Silicon carbide (SIC) material exists in many configurations known as polytypes. As a semiconductor material, one configuration, termed cubic SiC because of its cubic crystalline structure, has been the subject of worldwide research because it possesses the electron velocity versus electric field characteristics most suitable for microwave and millimeter wave power transistor devices. In the past, much of the research on the growth of cubic (3C) (beta) SiC thin films has been on silicon substrates. The resulting thin films of this earlier research were of poor quality, e.g., typically the thin films contained 10.sup.16 defects/cm.sup.2. The best of the devices represented by this earlier research is disclosed in U.S. Pat. No. 4,912,063 to Davis et al., entitled "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon". In the more recent past, methods have been developed to heteroepitaxially grow high quality cubic (3C) (beta) thin films of SiC on hexagonal (6H) (alpha) SiC substrates having a crystalline orientation on the basal plane (c-axis) or within .+-.0.10.degree. thereof. Thus, using these methods, the thin films produced were of comparatively high quality, e.g., typically these thin films contained only 10.sup.4 defects/cm.sup.2. Homojunction planar bipolar transistors have been formed on these films by using, inter alia, high temperature ion implantation of doping ions such that the base and emitter regions are formed therein. Using a similar but somewhat different approach, homojunction mesa bipolar transistors have also been formed on these films. The homojunction devices represented by this research are disclosed in U.S. Pat. No. 4,945,394 to Palmour et al., entitled "Bipolar Junction Transistor on Silicon Carbide".
In homojunction planar or mesa bipolar transistors as disclosed in Palmour et al., the transistor gain is derived from the ratio of the emitter doping concentration to the base doping concentration. For a current gain of 10, the base doping must be at least 10 times less than the emitter doping concentration. While this is tolerated in many applications, the lower base doping requirements lead to excessive base resistance and the resulting resistance times capacitance (RC) time constant of the emitter-base junction precludes operation at extremely high frequencies. In the homojunction bipolar devices of Palmour et al., both electrons and holes cross the emitter-base junction (in a typical ratio of 10:1). This leads to unwanted charge carrier storage effects in the emitter-base junction and these unwanted charge carriers inhibit the ability to rapidly turn off the transistor. Heterojunction bipolar transistors (HBTs) are well known in the III-V semiconductor materials wherein the material of the base is easily lattice-matched to the material of the emitter. The most common example being the use of an aluminum gallium arsenide (AlGaAs) emitter and a gallium arsenide (GaAs) base and collector. The efficacy of the npn HBT is that it provides virtually no barrier for electrons, but a large barrier for holes (or conversely in a pnp structure, virtually no barrier for holes and a large barrier for electrons). This is contrasted to a conventional homojunction bipolar transistor where the electron and hole barrier are essentially equal. In the HBT, charge carriers move in only one direction across the emitter-base junction resulting in an extremely large current gain--providing that there are few defects at the emitter-base junction to create interface charge states. The base in a HBT, unlike the base of the conventional homojunction bipolar transistor, can be impurity doped to a greater concentration than the emitter with negligible effect on gain. As such, the base resistance can be made to be very low and the device can operate at extremely high frequencies. While the use of SiC as a semiconductor material, as taught in Palmour et al., offers many advantages over conventional semiconductor materials in the fabrication of homojunction bipolar transistor devices, there is a need in the prior art to eliminate the excessive base resistance and the resulting resistance times capacitance (RC) time constant thereof in an improved manner.
As discussed in Palmour et al., the prior art is replete with examples of attempts to grow quality thin films, as well as attempts to fabricate junctions, diodes, transistors and like structures using SiC materials. SiC is difficult to work with because it occurs in more than 170 different one-dimensional ordering sequences without apparent variation in stoichiometry. As previously mentioned, these sequences or configurations are known as polytypes. It is also well known in the prior art that cubic (3C) SiC can be grown on [0001] oriented SiC wafers as described in the publication by Yoder et al,, entitled "Silicon Carbide Comes of Age" Naval Research Reviews, March 1989, pp. 26-33. Field effect transistors (FETs) have been fabricated in these resultant 3C SiC on 6H SiC films. As far as is known, in these and all other devices using 3C SiC on 6H SiC, the hexagonal 6H SiC substrate has not been electrically active. Hence, the 6H SiC substrate was merely used as a "holder" for the electrically active 3C SiC superstrate layers.
As discussed in Palmour et al. and Yoder e.t. al., aforementioned, as a semiconductor material, silicon carbide (SIC) offers many advantages over conventional semiconductors. Its chemical bonds are much stronger than those of conventional semiconductors thus rendering it a much harder material (less vulnerable to scratching during processing). These strong chemical bonds render its interatomic spacing (lattice constant) considerably shorter than that of conventional semiconductors. This hardness also suppresses the diffusion or migration of dopant impurities within the semiconductor. As such, the diffusion dominated wear out mechanism typical of III-V power transistors is absent in transistors made of SiC. The high relative hardness of SiC also renders it more immune to radiation damage. The dielectric constant of SiC is only 9.5 as compared to 11.8 for silicon, 12.8 for GaAs, and 14 for InP. This lower value significantly reduces the parasitic capacitance loss in SiC integrated circuits and extremely high frequency transistors. Unlike the conventional III-V semiconductors, SiC does not exhibit a region of negative electron mobility to reduce frequency response at high power. Stated differently, its charge carrier velocity is highest at the high values of electric field strength--exactly the property required for a combination of high power at high frequency operation.
To reiterate, the attributes of SiC have long been known; however, until recently SiC semiconductor material could not be produced with sufficient perfection to lead to viable production yields of electronic devices. For all of its superlative attributes, SiC suffers from the lack of other lattice-matched materials of its IV--IV semiconductor group. This is the reason, it is believed, that there has been no meaningful heterojunction activity in SiC devices. However, with the advent of single crystalline cubic (3C) SiC films grown on single crystalline hexagonal (6H) SiC wafers, this limitation can be overcome.
To amplify the above teachings, it is believed that the advantages of HBT transistors have not accrued to conventional stoichiometric SiC technology because there have been no suitable lattice-matched interface semiconductor materials. Attempts to fabricate devices by growing non-lattice-matched stoichiometric SiC emitters on lower bandgap silicon base and collector films are known in the prior art, but the severe lattice mismatch between these materials resulted in large interface state densities thereby rendering the devices virtually useless. Such devices (by virtue of the much lower breakdown voltage of silicon) are also not capable of providing the above-stated advantages of devices comprised solely of SiC. Non-stoichiometric SiC emitters on silicon base and collector structures including heterojunction bipolar transistor (HBT) structures have also been described in a U.S. patent application by Goodman et al., entitled "Trenched Bipolar Transistor Structures", U.S. Ser. No. 07/796,553, filed on Nov. 22, 1991, and assigned to the same assignee as the present application. These HBT structures, while overcoming the interface problem, greatly reduce the magnitude of the heterojunction offset potential and thereby limit the magnitude of the charge carrier velocity advantages that can be achieved in a complete SiC HBT structure. The non-stoichiometric approach also suffers the same disadvantages of the stoichiometric SiC on silicon device relating to breakdown voltages. Consequently, there is a need in the prior art to fabricate a silicon carbide (SIC) heterojunction bipolar transistor (HBT) structure having an electrically active heterojunction of cubic (beta) type SiC material on a substrate of higher bandgap hexagonal (6H) (alpha) type SiC material in an improved manner.
A silicon carbide (SIC) heterojunction bipolar transistor structure, which uses cubic (beta) SiC on hexagonal (alpha) SiC to form an electrically active heterojunction, according to the present invention, is believed an improvement over the prior art. This is so, inter alia, because the bandgap of cubic (beta) SiC is 2.2 eV while that of hexagonal (alpha) SiC is 3.0 eV, which permits a heterojunction offset potential of 0.4 eV between these two polytypes of SiC material. While the present structure has many attributes as previously disclosed, there is room in the prior art for improvements in at least three important areas, e.g., increase the variation in bandgaps (including larger heterojunction offset potentials), eliminate double positioning boundary defects and fabricate structures in an emitter region up configuration.
As further background, solid solutions of silicon carbide (SIC) are well known in the prior art. The alloy of silicon carbon aluminum nitrogen (SiCAlN) is believed to have the greatest potential as a semiconductor material. Single crystalline semiconducting thin films of SiCAlN are disclosed in U.S. Pat. No. 4,382,837 to Rutz, entitled "Epitaxial Crystal Fabrication of SiC:AlN". Various mole fraction ratios of SiCAlN were described in the publication by Dmitriev, entitled "SiC-Based Solid Solutions: Technology and Properties" Proc 3rd Int'l Conf on Amorphous and Crystalline Silicon Carbide and Other Group IV--IV Materials, Howard University, Washington, D.C., Apr. 11-13, 1990, pp. 1-21. Since aluminum nitride (AlN) is latticed-matched to silicon carbide (SIC), virtually any mole fraction ratio of this material can be synthesized with resulting bandgaps of 2.2 eV for cubic SiC through 6.2 eV for hexagonal (2H) AlN. This permits a heterojunction offset potention of 1.5 eV between these two compounds.
To continue, when SiC layers are grown epitaxially on 6H SiC substrates, the resulting layers can be either 6H, 3C, or mixture of 3C and 6H in crystalline structure. If the 6H substrate is cut such that its surface is &gt;3.0.degree. off the [0001] basal plane, then the resulting epitaxial layer is entirely of 6H structure. If the orientation is &lt;0.10.degree. off the [0001] basal plane, then the resulting epitaxial layer is entirely of 3C structure. As previously mentioned, this 3C structured material can be characterized by unwanted double positioning boundary defects.
Recently, epitaxial layers of SiCAlN have been grown on 6H SiC substrates oriented between 0.10.degree. and 3.0.degree. of the [0001] basal plane. In all cases (and unlike the 3C SiC on 6H SiC case), these films were of 3C crystalline structure and without double positioning boundary defects. This is truly an unexpected and as yet unexplained, but reproducible result. Growth was by atomic layer epitaxy (ALE) subjecting the 1120 Celsius 6H SiC surface in sequence to silane (SiH.sub.4), triethyl aluminum [(C.sub.2 H.sub.5).sub.3 Al], ethylene (C.sub.2 H.sub.4), and ammonia (NH.sub.3) after which the cycle was repeated layer by layer. The bandgap of such material is from 2.2 eV to 5.2 eV depending on the mole fraction ratio between the SiC and the AlN constituent parts (Since this is 3C material, the maximum bandgap is 5.2 rather than 6.2 eV for pure 3C AlN.) At mole fractions of AlN exceeding 40.3%, the bandgap of the SiCAlN exceeds that of the 6H SiC substrate on which it is grown while with lower mole fractions, the bandgap of the resulting epitaxial layer is less than that of the underlying 6H SiC substrate.
Epitaxial films of SiCAlN grown by ALE on 6H SiC substrates are of a quality superior to those of 3C SiC films grown on 6H SiC substrates. Moreover, and unlike 3C epitaxial films of pure SiC, the bandgap is controllable between 2.2 and 5.2 eV thereby rendering these films efficacious for heterojunction devices and particularly so for heterojunction bipolar transistors. This efficacy derives from (1) the absence of significant strain at the interface between the 6H SiC substrates and the overlying SiCAlN epitaxial films, (2) the relative similarity of bond angles between the substrate and the epitaxial overlayer thereby reducing the possibility of interface electronic traps that are deleterious to electron devices and (3) the absence of double positioning boundary defects in the overlying SiCAlN layers. Consequently, there is a need in the prior art to fabricate an improved heterojunction bipolar transistor using a solid solution of silicon carbide such as SiCAlN, according to the present invention.